I ended off yesterday’s post with the question why we don’t say ee avid lo mehanei by the case of selling land latzmisus, for perpetuity. In the case of someone who covets an object and forces the owner to sell it, violating lo tachmod, we explained that ee avid lo mehanei doesn’t apply because the issur is in the coveting, the desire -- the actual sale is just an expression, a siman, of the issur, not the issur itself. However, with respect to the issur of lo timacheir latzmisus it seems clear that the sale itself is the act which the Torah prohibits.
PC already suggested a sevara in a comment, and while I don’t think it was the Magid Mishne’s intent (see yesterday’s post), the sevara is nonetheless a good one. Let me introduce it with a different question (raised in Shu”T R’ Akiva Eiger): why is meat shechted on Shabbos (first offense of the shochet) kosher? Since the Torah prohibits shechita on Shabbos, why not say ee avid lo mehanei?
The answer to the question becomes clear if we reframe it in a different context. If you build a house on Shabbos, would you say ee avid lo mehanei because you violated the melacha of boneh? That makes no sense – the house is standing, it is a metziyus; ee avifd lo mehanei cannot deny or undo reality. Similarly, shechita is nothing more than a metziyus, a reality, of meat which has been slaughtered properly. No amount of legal grandstanding can undo reality. (I'm just using this case to make a point, so let’s save for another time the discussion of whether this answer is true, i.e. whether shechita is in fact simply a metziyus or whether it is a din, a heter achila, that results from a metziyus.)
We once discussed why the rule of aseh doche lo ta’aseh doesn’t apply to dinei mamonos (link) and we raised the following question: is ownership a result of the fact that there is an issur of gezel on anyone else taking an object, or is the issur of gezel a result of the existance of ownership? R' Shimon Shkop opts for the latter definition. First clarify the rules of the marketplace, what ownership means, how property is acquired, etc. and then we can apply the rules of issur v'heter and discuss whether gezel applies.
Ee avid lo mehanei can undo a halachic construct, but it cannot undo reality. Commercial law creates its own reality – a sale is a sale by virtue of the rules of the marketplace. Whether a sale violates lo tahcmod, or mechira latzmisus, in either case the sale stands and cannot be undone.